London mayor Ken Livingstone has stepped into the Gatwick Airport row by backing plans for massive expansion of the airport.

Mr Livingstone, a former Sussex resident, said the capital's economy was growing so fast that extra airport capacity will be essential within the next 20 years.

But he ruled out any further development of Heathrow or plans to build an entirely new airport at Cliffe, in Essex.

The mayor said this left Gatwick, or Stansted airport in Essex, as the possible solutions to the Government's transport headache.

His office also revealed the findings of a privately commissioned poll showing 47 per cent of London residents are in favour of more runways, with only 23 per cent against.

Stansted won the most backing, with 42 per cent in favour of a new runway and 19 per cent against, followed by Gatwick, with 40 per cent in favour and 21 per cent against.

Mr Livingstone said: "Rigorous environmental standards and adequate public transport are absolute preconditions."

A 1979 agreement promised there would be no new runways at Gatwick for at least 30 years.

Anti-Gatwick expansion campaigner Brendan Sewell said today: "Mr Livingstone needs to explain how he thinks the Government could overturn a binding legal agreement.

"Surely he is not in favour of the Government breaking the law?"